The present inventor developed an equipment for measurement of neuro-magnetic field above human heads with a SQUID cryogenically cooled by liquid helium and used as a magnetic sensor. The instrument has been practically used.
Referring to FIG. 4, the conventional MagntoEncephaloGraphic equipment (MEG equipment) in a superconducting magnetic shield comprises a thermal insulator structure of hollow vacuum cylinder 11, a refrigerator unit for circulating Helium gas 12, a cryogenic vessel 13 and a top cover 14. The thermal insulator structure 11 is a double-walled, hollow vacuum cylinder comprising a first enclosure of high critical temperature superconductor 111 and a second enclosure 112 of a high-permeability magnetic material both concentrically arranged in its double cylindrical wall. The refrigerator unit for circulating Helium gas 12 circulates a cooling medium or coolant to cool the first, high critical temperature superconductor enclosure 111 placed in the annular space of the double-wall. The first enclosure 111 has a line cooling pipe wound therearound, and a helium gas is circulated in the cooling pipe to cool the enclosure 111, thereby preventing the surrounding magnetic field from invading the inside of the hollow vacuum cylinder 11. It should be emphasized in FIG. 4 of the conventional MEG equipment that the cryogenic vessel 13 is also a double-walled, hollow vacuum cylinder with a vacuum bottom. In other words, the conventional magnetoencephalographic equipment has two separate hollow vacuum cylinders 11 and 13, which is completely different from the present invention as mentioned later.
The cryogenic vessel 13 is fixedly located in the inside of the hollow vacuum cylinder 11. The top cover 14 is a double-layer enclosure rf-shielded by a metal of good conductivity (electromagnetic wave shield) and shielded magnetically by a magnetic material (magnetic field shield) to fit the top of the hollow vacuum cylinder 11. The lower part of the cryogenic vessel 13 defines a head accommodating area 131 to encircle the head of a test subject. A plurality of SQUID magnetic sensors 15 are mounted on a sensor block 20, which is arranged in the cryogenic vessel 13, spreading over the head accommodating area 131. The cryogenic vessel 13 is filled with a cryogenic coolant or liquid helium
The hollow vacuum cylinder 11 stays on the floor with a chair of non-magnetic material 17 arranged in its lower open end. The top cover 14 of magnetic material effectively prevents the electromagnetic wave and the magnetic field of the earth from invading the hollow vacuum cylinder 11. The part of the cap enclosure 14 is removed when one supplies the cryogenic vessel 13 with liquid helium using a transfer tube.
The following documents show the conventional art described above:    Patent Document 1: Patent Application Laid-Open No. H10-313135;    Patent Document 2: International Publication Number WO2004/066836 A1    Non-Patent Document 1: “Whole-Head-Type SQUID System in a Superconducting Magnetic Shield of High Critical-Temperature Superconductor”, by Hiroshi Ohta, the magazine, “Ceramics 35” (2000), No. 2, Extra Edition, sub-titled “Brain and Ceramics; Ceramics Useful in Studying Functions of Human Brains, Making the Diagnosis of the Brain Disorders and Carrying out Required Treatments”; and Non-Patent Document 2: “Nanometer SNS Junctions and Their Application to SQUIDs” by Hiroshi Ohta et al the magazine, “PHYSICA C” 352 (2001), pages 186-190.
The hollow vacuum, cylinder 11 and the cryogenic vessel 13 are described below in detail When the first enclosure 111 of high critical-temperature superconductor (bismuth-strontium-calcium-copper-oxide: BSCCO) is cooled down to the vicinity of the liquid nitrogen temperature (103K or below), no magnetic flux is allowed to invade the inside of the hollow vacuum cylinder 11 from the exterior. Before the temperature of the first enclosure 111 is lowered, there already stays a significant amount of magnetic flux due to the magnetic field of the earth in the inside of the hollow vacuum cylinder 11. The magnetic flux due to the magnetic field of the earth is trapped in the first enclosure 111 when the temperature is lowered below the critical temperature of BSCCO. The axial or radial movement of the cryogenic vessel 13 will change the relative displacement between the component of the trapped magnetic field and the pick-up coils of the SQUID magnetic sensors generating noise outputs in the measurement. The way to eliminate such noise outputs is to: keep the relative displacement between the hollow vacuum cylinder 11 and the cryogenic vessel 13 constant; and mechanically isolate the hollow vacuum cylinder 11 from the floor on which the magneto encephalographic equipment stands, thereby blocking any mechanical vibrations which otherwise would be transferred to the hollow vacuum cylinder 11 from the floor. Thus, the magneto encephalographic equipment is guaranteed to be free of change of relative displacement between the cryogenic vessel 13 and the first, enclosure 111 in the hollow vacuum cylinder 11 (see Patent Document 2). Specifically pillow blocks are used to fill the space between the inner wall of the hollow vacuum cylinder 11 and the outer wall of the cryogenic vessel 13, and the hollow vacuum cylinder 11 is fixed to the floor via a vibration-isolation support, which comprises mechanical dumpers and an active vibration isolation unit. Such vibration isolation method is quite useful in effectively removing noises if any during the measurement of the neuro-magnetic field and improving Signal-to-Noise ratio of magnetoencephalography data.
Our goal is to make a maintenance-free operation of a MEG equipment using an available power of a commercial closed-cycle helium refrigerator. Our preliminary experiment with a commercial closed-cycle helium refrigerator teaches that the heat flow through the vacuum thermal insulation between the two walls of the double-layer cryogenic vessel 13 beats the available cooling power of a commercial closed-cycle helium refrigerator and that the level of the liquid helium inside the cryogenic vessel decreases gradually.
Now, what remains unsolved is to minimize the heat flow into the cryogenic vessel 13 so that the available cooling power of a commercial closed-cycle helium refrigerator can manage to keep the level of liquid helium constant in the cryogenic vessel 13.
It is practically impossible from both points of design's view and maintenances view that the cooling power of a commercial helium refrigerator beats the heat flow into the cryogenic vessel 13 in the conventional configuration as shown in FIG. 4.
Details are following. The cryogenic vessel 13 contains approximately 40 liters of liquid helium (−270° C.), which constantly evaporates and changes into a helium gas, discharging into the atmosphere while the measuring equipment works. Approximately 20 liters of liquid helium evaporates every day, and therefore, the equipment needs to be supplied with 20 liters of liquid helium every day. The running cost due to liquid helium consumption is approximately ten million yen per year. The cryogenic liquid must be handled with considerable care and otherwise it might boil explosively, and therefore, some experts capable of handling such cryogenic liquid need to be retained. In the hope of solving these maintenance problems it is proposed that helium gas is used as a substitute for the liquid, helium; the gas can be pumped and circulated from the cryogenic refrigerator. If the conventional configuration as shown in FIG. 4 is not changed and the heat flow into the cryogenic vessel 13 is not reduced, then the required capacity or cooling power of the closed-cycle helium refrigerator must be huge and impractical. As a matter of fact a refrigerator whose capacity is large enough to provide such a cooling capacity is too large to be produced, installed and operated. The helium gas circulating refrigerator system is barely able to meet the cooling of the magnetic shield enclosure 111 at a temperature of liquid nitrogen temperature (−200° C.) much higher than the liquid helium temperature—the temperature at which the cryogenic vessel 13 needs to be cooled. Therefore there is no other way than managing to reduce the heat flow into the cryogenic vessel 13 some way. In this connection the liquid and gas cooling modes are being selectively used at present: the cryogenic vessel 13 is liquid-cooled while being constantly supplied with as much liquid helium as evaporated whereas the first enclosure 111 of high critical temperature superconductor located within the double wall is gas-cooled by circulating helium gas.